user
You need to complete the following task:

For the following sentence, identify and explain the grammatical error.

This is a book that I readed.

Answer:
assistant
The grammatical error in the sentence is a verb conjugation error. The correct verb for the sentence should be 'read' instead of 'readed'. 'Readed' is an incorrect form of the verb 'read', which is conjugated from the base verb 'read' in the past simple tense.